We report absolute cross sections for K-and L-shell ionization of silver and gold targets, respectively, by lepton (electron, positron) impact in the threshold region. The experiments were performed at the slow positron source TEPOS facility at the linac of the Strahlenzentrum in Giessen. A different behavior of positron and electron impact near threshold was observed. In comparison with theoretical calculations, our results suggest that the differences between electron and positron impact are mainly caused by the acceleration or deceleration of the incident projectile in the target nuclear field.PACS numbers: 34.80.Dp, 34.50.FaThe interaction of an incident projectile with a target atom may give rise to a variety of different elastic and inelastic scattering processes. Ionization of, for example, inner atomic shells by lepton (positron, electron) impact is one of the processes being of fundamental importance for our understanding of collision dynamics. Experimental investigations for electron impact have been reported by many authors (e.g., Ref.[1] and references therein). For positron impact, few absolute cross sections have been reported yet; they were restricted to incident energies Eo> 100 keV (i.e., EQ/I><\, where / is the innershell binding energy) [2,3]. No differences between positron and electron impact were reported in these former experiments. In this paper we report first absolute cross sections for inner-shell ionization by positron (e + ) impact in the threshold regime. We compare these results with cross sections for electron (e~) impact which we have measured with the same apparatus. In the threshold region, we expect different cross sections for positron and electron impact for two reasons: the exchange effect (Ex), important in the ionization by electron impact, and the Coulomb effect (C); the latter effect is caused by the projectile-target nucleus interaction and results in a slowing down (e + ) or acceleration (e ~ ) of the incident projectile.Details of the experimental setup have been described before [4]. Very briefly, it consists of the electron accelerator-based slow positron source TEPOS which delivers intense positron beams of about 6x \0 1 e "Vsec. Positrons with kinetic energy of 2 keV were extracted from the source; they were postaccelerated by a suitable high voltage applied to the target [5,6]. For the measurements with electrons, a corresponding gun was installed in the beamline. The incident energy of the projectiles was varied from 30 keV up to 70 keV for positrons and from 12.3 keV up to 75 keV for electrons. The measurements were performed by detecting characteristic x rays resulting from the inner-shell ionization process. A Si (Li) detector was installed perpendicular to the incident beam. As target we used thin silver (950-5000 A) and gold (800-1000 A) monolayer foils and gold/silver (Au 600 A/Ag 1000 A, Au 1000 A/Ag 1800 A, and Au 1450 A/Ag 3500 A on a carbon backing) multilayer foils. The purpose of these multilayer foils was to measure simultaneously the x-ray yield of...

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The slow positron beam facility TEPOS at the 65 MeV electron linear accelerator in Giessen has been used to produce an intense beam of monoenergetic positrons. The impact energy of the positrons hitting the surface of a polycrystalline molybdenum foil has been varied between 0 and 470 eV. We observed the formation of n = 2 positronium by the emitted Lyman-a radiation and its change as a function of the positron energy. We found a maximum yield of excited positronium at an implantation energy of (60 i 20) eV. The yield decreased to 50% of the maximum value at about 200 eV.

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